An extraction with 90% acetone the conditions described in the method which follows has been considered satisfactory by most workers for many years. We believe this still to be the case for most seawater samples, having regard to the accuracy considered adequate for most investigations of marine ecology. However, results are undoubtedly low in many instances because of the presence of plant cells that are not fully extracted. With certain species 50% or more of the pigments may be left behind in the cell. A change of solvent may be beneficial but will rarely ensure complete extraction so it is probably not worth the trouble to re-establish extinction coefficients for other solvents or solvent mixrures.The use of a sonic disintegrator has been recommended but we have not found sufficient improvement to merit the application of such equipment on a routine basis. The use of a tissue grinder, such as that recommended by Yentsch and Menzel (Deep-Sea Res., 10: 221,1963), is relatively convenient and improves results on many natural population but even this approach fails to give complete extraction in a reasonable time with certain species. Fortunately bad cases are generally found only among the Chlorophyta and some benthic species, and reasonable results will be obtained with open-sea samples for much of the time, even without grinding. For work in lakes or shallow estuaries, the adequacy of extraction must be carefully checked.